The present invention relates to binders and radiation-sensitive compositions comprising said binders and, Inter alia, are excellently suitable for the production of lithographic plates.
Currently, photosensitive compositions usable particularly for high-performance lithographic plates must fulfill high requirements.
In order to improve the properties of photosensitive compositions and thus also of the corresponding lithographic plates, essentially two different ways are taken. One of them deals with the improvement of the properties of the photosensitive components in the compositions (frequently negative diazo resins, photo polymers etc.), the other one with the search for novel polymeric compounds (xe2x80x9cbindersxe2x80x9d), which are to control the physical properties of the photosensitive layers. In particular the latter way is decisive for lithographic plates because the behavior in the developing and printing processes (such as developability, ink receptivity, scratch resistance, consistency in the number of prints produced) is decisively influenced by the polymeric binders. Also shelf life and photosensitivity of the materials are strongly influenced by such polymeric compounds.
The polymeric binders, therefore, exhibit various structural elements for satisfying the extensive requirements, which may have different effects on individual properties. For instance, hydrophilic structural elements such as carboxyl groups, hydroxyl groups and the like generally promote the developability of the photosensitive compositions in aqueous alkaline developers and partly ensure sufficient adhesion to polar substrates. Hydrophobic structural elements, on the other hand, reduce the capability of being developed in the above-mentioned developers, but ensure the good ink receptivity used in the printing process, which is indispensable in lithographic plates.
Due to the broad range of requirements regarding the polymeric binders, there have been extensive studies for many years on the synthesis and optimization of the use of these substances for photosensitive compositions, cf. e.g. H. Baumann and H.-J. Timpe: xe2x80x9cChemical Aspects of Offset Printingxe2x80x9d in J. prakt. Chem./Chemiker-Zeitung [Journal for chemists] 336 (1994) pages 377-389.
Negative photosensitive compositions for lithographic plates can be divided into the following two main groups:
a) Systems where the binder forms a network together with a photosensitive component due to exposure to light. Such photosensitive components are e.g. diazo resins; reactive photolysis products thereof react for example with OH groups of the binder. A disadvantage of these systems is their low print run stability.
b) Systems containing binders with photoreactive groups, which can be crosslinked by exposure to light (in many cases by the additional use of a sensitizer). These systems allow the production of plates with high print run stability.
The following applications and patents deal with systems of item b) above: U.S. Pat. No. 2,690,966 discloses cinnamic acid esters of polyvinyl alcohol useful in photomechanical resist compositions. GB-B-1274017 describes lithographic printing plates comprising a layer containing polyvinylacetal; the polyvinylacetal contains at least 10 wt.-% polyvinyl alcohol units which are partially esterified with cinnamic acid. These printing plates are, however, only developable with developers having a high solvent content which is disadvantageous for environmental reasons.
In U.S. Pat. No. 2,861,058 radiation-sensitive polymers are claimed where polyvinylalcohol is esterified with cinnamic acid and further acids and contains free carboxyl groups in the side chain of the polymer. Printing plate precursors comprising such a polymer are, however, insufficient in photospeed.
In EP-A-0 092 901 polyvinyl acetals modified by chalcon groups are claimed. The length of run of plates made with these polymers and the photospeed are, however, insufficient nowadays.
U.S. Pat. No. 3,030,208 and U.S. Pat. No. 3,622,320 disclose polymers which contain cinnamoyl groups in the main chain of polyester resins. The developing of radiation-sensitive elements containing such polymers requires the use of developers with high solvent content; furthermore, there is a high tendency to form sludge in the developing machine.
In U.S. Pat. No. 3,929,489, and U.S. Pat. No. 4,419,437 polymers are claimed which contain cinnamoyl groups in the main chain of polyester resins and further contain disulfone amino groups. The developability and length of run of plates obtained therewith, however, do not meet the high requirements nowadays.
In U.S. Pat. No. 2,787,546 acetal polymers are disclosed containing in one side chain both cinnamate and carboxylic group functionalities. However, plates comprising such polymers suffer from the disadvantage of insufficient photospeed.
It is thus the object of the present invention to provide polymers for radiation-sensitive compositions which, in addition to their high photospeed and resolution, ensure a high durability on the press and also a low tendency to form sludge in the developing processor during developing and which, furthermore, do not require developers with a high content of organic solvents.
These objects are achieved by means of the cinnamic acid groups containing acetal polymers comprising units A-D and optionally E, wherein A is present in an amount of about 25 to 55 mol % and is of the formula 
B is present in an amount of about 0.5 to 25 mol % and is of the formula 
wherein R1 is selected from the group consisting of alkyl, aryl and aralkyl,
C is present in an amount of about 0.5 to 40 mol % and is of the formula 
wherein X is an aliphatic or aromatic spacer group, and Ac is an acidic group selected from the group consisting of xe2x80x94COOH, xe2x80x94SO3H or xe2x80x94SO2NR9R10 with R9 and R10 independently being selected from hydrogen and alkyl; and when X is phenylene, Ac can also be OH,
D is present in an amount of about 20 to 70 mol % and is of the formula 
wherein Ci is selected from the group consisting of Ci-1, Ci-2, Ci-3 and Ci-4 which are represented by the following formulae 
wherein:
k, m and n are integers independently selected from 0 to 5, R3, R4 and R7 are independently selected from the group consisting of alkyl, alkoxy, xe2x80x94COOR8,
xe2x80x94NR9R10, xe2x80x94NHxe2x80x94COxe2x80x94CH3, halogen and cyano,
R8 is selected from hydrogen and alkyl;
R9 and R10 are independently selected from hydrogen and alkyl,
R5 is selected from the group consisting of hydrogen, alkyl, aryl and aralkyl;
R6 is selected from the group consisting of alkyl, aryl and aralkyl and
Y is selected from the group consisting of alkylene, arylene and arylenealkylene; and
E is present in an amount of about 0 to 50 mol % and is of the formula 
wherein R2 is selected from the group consisting of alkyl, aryl and aralkyl groups.
The present invention relates to photosensitive compositions comprising:
(i) a polymeric binder and
(ii) optionally at least one other component selected from the group consisting of photosensitizers for photocyclo addition, diazo resins, colorants, exposure indicators, surfactants, stabilizing acids and plasticizers
characterized in that the binder comprises units A, B, C, D and optionally E wherein A is present in an amount of about 25 to 55 mol % and is of the formula 
B is present in an amount of about 0.5 to 25 mol % and is of the formula 
wherein R1 is selected from the group consisting of alkyl, aryl and aralkyl
C is present in an amount of about 0.5 to 40 mol % and is of the formula 
wherein X is an aliphatic or aromatic spacer group, and Ac is an acidic group selected from the group consisting of xe2x80x94COOH, xe2x80x94SO3H or xe2x80x94SO2NR9R10 with R9 and R10 independently being selected from hydrogen and alkyl; and when X is phenylene, Ac can also be OH,
D is present in an amount of about 20 to 70 mol % and is of the formula 
wherein Ci is selected from the group consisting of Ci-1, Ci-2, Ci-3 and Ci-4 which are represented by the following formulae 
wherein:
k, m and n are integers independently selected from 0 to 5, R3, R4 and R7 are independently selected from the group consisting of alkyl, alkoxy, xe2x80x94COOR8;
xe2x80x94NR9R10, xe2x80x94NHxe2x80x94COxe2x80x94CH3, halogen and cyano;
R8 is selected from hydrogen and alkyl;
R9 and R10 are independently selected from hydrogen and alkyl;
R5 is selected from the group consisting of hydrogen, alkyl, aryl and aralkyl;
R6 is selected from the group consisting of alkyl, aryl and aralkyl and
Y is selected from the group consisting of alkylene, arylene and arylenealkylene, and
E is present in an amount of about 0 to 50 mol % and is of the formula 
wherein R2 is selected from alkyl, aryl and aralkyl groups.
Unless defined otherwise, within this disclosure the terms xe2x80x9calkylxe2x80x9d, xe2x80x9calkylenexe2x80x9d, xe2x80x9chalogenxe2x80x9d, xe2x80x9carylxe2x80x9d, xe2x80x9carylenexe2x80x9d, xe2x80x9caralkylxe2x80x9d, xe2x80x9carylenealkylenexe2x80x9d and xe2x80x9calkoxyxe2x80x9d have the following meanings:
The term xe2x80x9calkylxe2x80x9d as used herein includes straight chain alkyl groups and branched chain alkyl groups; preferably the alkyl group consists of 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms. The alkyl groups can be unsubstituted or substituted with at least one substituent like OH and halogen.
As used herein the term xe2x80x9chalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d includes fluorine, chlorine, bromine and iodine atoms.
The term xe2x80x9calkoxyxe2x80x9d includes straight chain alkoxy groups as well as alkoxy groups wherein the alkyl moiety is a branched chain alkyl moiety; preferably the alkoxy group has 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms.
The term xe2x80x9carylxe2x80x9d includes unsubstituted aryl groups and aryl groups having 1 to 3 substituents selected from the group consisting of halogens, alkyl, alkoxy, acyl, amino group, cyanide group and nitro group; preferably an aryl group consists of 6 to 14 carbon atoms in the aryl moiety (i.e. without optional substituted), more preferably the aryl group is substituted or unsubstituted phenyl or naphthyl, especially preferred unsubstituted phenyl or naphthyl. The term xe2x80x9carylxe2x80x9d also includes 5- and 6-membered heterocyclic aryl groups.
The term xe2x80x9caralkylxe2x80x9d refers to a group having an aryl moiety as defined above and an alkyl moiety as defined above.
The term xe2x80x9carylenealkylenexe2x80x9d refers to a divalent xe2x80x9caralkylxe2x80x9d group wherein the aryl and alkyl moieties are as defined above.
The term xe2x80x9calkylenexe2x80x9d refers to a divalent xe2x80x9calkylxe2x80x9d group wherein the alkyl moiety is as defined above. The term xe2x80x9carylenexe2x80x9d refers to a divalent xe2x80x9carylxe2x80x9d group wherein the aryl moiety is as defined above.
Unit A is present in the copolymer of the present invention in an amount of about 25 to 55 mol %, preferably about 30 to 40 mol %.
Unit B amounts to about 0.5 to 25 mol % in the copolymer of the present invention, preferably about 2 to 15 mol %. R1 is selected from the group consisting of alkyl, aryl and aralkyl; especially preferred R1 is alkyl, most preferred methyl. Unit B can be present several times in the copolymer with various moieties R1 independent of one another.
The copolymer of the present invention contains about 0.5 to 40 mol % of unit C, preferably about 2 to 20 mol %. In formula (C) X is an aliphatic or aromatic spacer group, preferably C1-C8 alkylene or C6-C14 arylene, which can be substituted as defined above for xe2x80x9calkylxe2x80x9d and xe2x80x9carylxe2x80x9d. Especially preferred X is arylene and most preferred 1,4-phenylene. Ac is selected from the group consisting of xe2x80x94COOH, xe2x80x94SO3H and xe2x80x94SO2NR910 (R9 and R10 are independently selected from hydrogen and alkyl), preferably xe2x80x94COOH and xe2x80x94SO2NR9R10, most preferred xe2x80x94COOH; if X is phenylene Ac can also be OH.
Unit C can be present several times in the copolymer with various moieties X and Ac independent of one another.
Unit D is present in the copolymer of the present invention in an amount of about 20 to 70 mol %, preferably about 50 to 60 mol %. Ci is selected from Ci-1, Ci-2, Ci-3 and Ci-4 represented by the following formulae 
k, m and n are integers independently selected from 0 to 5, preferably 0 to 3, most preferred 0 and 1.
R5 is selected from the group consisting of hydrogen, alkyl, aryl and aralkyl, preferably alkyl, most preferred methyl. R3, R4and R7 are independently selected from the group consisting of alkoxy, alkyl, xe2x80x94COOR8, xe2x80x94NR9R10, halogen, cyano and xe2x80x94NHxe2x80x94COxe2x80x94CH3, with R8, R9 and R10 being independently selected from hydrogen and alkyl (preferably alkyl); preferably R3, R4 and R7are selected from the group consisting of alkyl and alkoxy.
Y is selected from the group consisting of alkylene, arylene and arylenealkylene (the aryl or the alkyl moiety thereof can be bonded to 
), preferably alkylene, most preferred xe2x80x94(CH2)zxe2x80x94 with z being an integer selected from the group consisting of 1, 2 and 3.
R6 is selected from the group consisting of alkyl, aryl and aralkyl, preferably alkyl. The copolymer of the present invention contains at least one kind of formulae Ci-1, Ci-2, Ci-3 and Ci-4. Furthermore, each moiety of Ci-1, Ci-2, Ci-3 and Ci-4 can be present several times in a copolymer with various R3 to R7 independent of one another. The particularly preferred Ci group is Ci-1 with k=0.
Optionally the copolymer of the present invention further contains unit E in an amount of up to 50 mol %; it is however preferred that the copolymer does not contain unit E. If unit E is present in the copolymer it can be present in an amount of about 1 to 25 mol % and furthermore, can be present several times with various R2 independent of one another. R2 is selected from the group consisting of alkyl, aryl and aralkyl; preferably it is selected from the group consisting of C1-C5 alkyl.
The vinyl alcohol/vinyl acetate copolymers that serve as starting material for the preparation of the polymers of the present invention are 70 to 98 mol % hydrolyzed and have a weight-average molecular mass Mw of 20,000 to 130,000 g/mol. Suitable copolymers of this type are commercially available. Which copolymer is used as starting material for the synthesis depends on the intended use of the photosensitive composition. If offset lithographic plates are to be produced, polymers with a weight-average molecular mass Mw of 35,000 to 130,000 g/mol and an hydrolysis degree of the vinyl acetate structural unit of 80 to 98 mol % are preferably used.
The preparation of the copolymers of the present invention takes place in two synthesis steps. In a first step, monomeric aldehydes/acetals containing a cinnamoyl group are prepared which in a subsequent second step are reacted with polyvinyl alcohol to obtain the polyvinyl acetals containing cinnamoyl groups in the side chains. The two steps may be carried out sequentially with or without isolating the aldehydes/acetals.
For obtaining a polyvinyl acetal with unit D being Ci-1 the following aldehyde (I) has to be prepared in the first step 
wherein R3 and k are as defined above.
Aldehyde (I) can be obtained by reacting hydroxybenzaldehyde with the corresponding cinnamoyl chloride in the presence of an amine catalyst. For obtaining a polyvinyl acetal with unit D being Ci-2 the following acetal (II) has to be prepared in the first step 
wherein R is alkyl, and m, Y, R4 and R5 are as defined above.
Acetal (II) can be obtained by reacting the corresponding cinnamoyl chloride with the corresponding amino substituted aldehyde dialkyl acetal.
For obtaining a polyvinyl acetal with unit D being Ci-3 the following acetal III has to be prepared in the first step 
wherein R, R5 and R6 are as defined above.
Acetal (III) can be obtained by reacting the corresponding isocyanatocinnamic acid ester with the corresponding amino substituted aldehyde dialkyl acetal.
For obtaining a polyvinyl acetal with unit D being Ci-4 the following aldehyde (IV) has to be prepared in the first step 
wherein n and R7 are as defined above.
Aldehyde (IV) can be obtained by acylation of benzaldehyde with the corresponding cinnamoyl chloride.
When preparing the binders of the present invention in organic solvents or hydroxyl-group containing solvent/water mixtures, the end products are precipitated by stirring them into water. The precipitation procedure may also take place such that water is stirred into the reaction mixture. In both cases, the mixture must be intensely mixed in order to obtain a reaction product easy to handle and to separate the side products that also form as quantitatively as possible. In reactions in water, the product will precipitate in the course of the reaction. In both cases, the precipitated reaction product is separated, washed with water and then dried using hot air of 45 to 50xc2x0 C. until the water content is reduced to no more than 3 wt.-%.
In the radiation-sensitive composition of the present invention the copolymer containing units A-D and A-E, respectively, as defined above is preferably present in an amount of about 10 to 95 wt.-% based on the solid content of the composition, more preferably about 30 to 85 wt.-%.
The radiation-sensitive composition of the present invention can further contain a photo-sensitizer for photocyclo addition. The use of a photosensitizer can be advantageous if the wavelength used for imaging the radiation-sensitive element of the present invention is outside the wavelength range where the cinnamate absorbs. All sensitizers enabling a photocyclo addition known in the field of the art may be used. Examples are, for instance, the sensitizers mentioned in DE-A-26 26 769, DE-42 31 324 and U.S. Pat. No. 5,476,754 such as xanthones, thioxanthones, aceto phenone, benzaldehyde, carbazole, triphenyl amine, hexachloro benzene, 4,4-diphenyl cyclohexadienone, 1,2-dibenzoyl benzene, benzophenone and derivatives thereof, 1,4-diacetyl benzene, fluorene, anthrone, benzanthrone and derivatives thereof, 2-nitrofluorene, quinoxaline, 4-nitrobiphenyl, 4-cyano benzophenone, thioxanthone (also alkyl or halo substituted), phenyl glyoxal, anthraquinone, quinoline, phenantrene, flavone, Michler""s ketone, 4-acetyl diphenyl, 2-acetonaphthene, acridine yellow, 1-naphthyl phenyl ketone, chrysene, 1-acetonaphthol, 1-naphthaldehyde, coronene, benzil, fluorenone, fluorescein (acid), aromatic nitro compounds such as p-nitrostilbene, 5-nitro acenaphthene and 4-nitroaniline, naphthothiazoline and derivatives thereof, 1-acetylamino-4-nitronaphthaline, quinones, benzo thiazoline derivatives, naphtho thiazole derivatives, ketocumarin derivatives, benzothiazole derivatives, naphthofuranone compounds, benzothiazolines, pyrylium salts and thiapyrylium salts.
Particularly preferred are ketocumarins, thioxanthones and benzophenones; especially preferred thereof are isopropyl thioxanthone, chlorothioxanthene and Michler""s ketone.
The weight portion of the sensitizers is preferably about 0.5 to 20 wt.-% based on the solid content of the photosensitive composition, more preferably about 1 to 15 wt.-% and most preferably about 1 to 10 wt.-%.
The radiation-sensitive composition of the present invention can further comprise at least one other additive selected from the group consisting of colorants, exposure indicators, stabilizing acids, plasticizers and surfactants. The total amount of these additives is preferably about 0.25 to 25 wt.-% based on the solid content of the radiation-sensitive composition, more preferably about 1 to 20 wt.-%.
Suitable colorants are those that dissolve well in the solvent or solvent mixture used for coating or are easily introduced as a pigment in the disperse form; they include inter alia rhodamin dyes, methyl violet, anthraquinone pigments and phthalocyanine dyes and/or pigments. The dyes and/or pigments may be comprised in the photosensitive composition in an amount of about 1 to 15 wt.-%, more preferably about 2 to 7 wt.-%.
Furthermore, the composition of this invention may comprise stabilizing acids. These acids include phosphoric, citric, benzoic, 4-toluene sulfonic and tartaric acid. In some formulations, a mixture of several different acids is advantageous. 4-toluene sulfonic acid is preferably used as acid. The added acid preferably amounts to about 0.2 to 3 wt.-%.
Furthermore, the photosensitive composition of this invention may comprise more components, such as plasticizers. Suitable plasticizers include dibutyl phthalate, triaryl phosphate and dioctyl phthalate. Dioctyl phthalate is especially preferred. The amount of plasticizer used is preferably about 0.25 to 2 wt.-%.
The composition of this invention may also comprise exposure indicators which indicate exposure to radiation by changing their color. Suitable exposure indicators are for instance metanil yellow and 4-phenylamino azo benzene. The indicators are preferably used in an amount of about 0.3 to 5 wt.-%.
Also diazo resins usually used in negative working radiation-sensitive compositions may be present in the photosensitive compositions of the present invention. If present, their amount is preferably about 1 to 50 wt.-% based on the solid content of the composition, more preferably about 1 to 40 wt.-%.
The diazo resin is a condensation product of a monomeric diazonium compound and a second partner. Such condensation products may for instance be prepared in a common manner by condensation of a diazo monomer such as 4-diazo-diphenylamine, 1-diazo-4-N,N-dimethylamniobenzene, 1-diazo-4-N,N-diethylaminobenzene, 1-diazo-4-N-ethyl-N-hydroxyethyl-aminobenzene, 1-diazo-4-N-methyl-N-hydroxyethylamino-benzene, 1-diazo-2,5-diethoxy-4-benzoylaminobenzene, 1-diazo-4-N-benzylaminobenzene, 1-diazo-4-N,N-dimethylaminobenzene, 1-diazo-4-morpholinobenzene, 1-diazo-2,5-dimethoxy-4-p-tolylmercaptobenzene, 1-diazo-2-ethoxy-4-N,N-dimethylaminobenzene, p-diazo-dimethylaniline, 1-diazo-2,5-dibutoxy-4-morpholinobenzene, 1-diazo-2,5-diethoxy-4-morpholinobenzene, 1-diazo-2,5-dimethoxy-4-morpholinobenzene, 1-diazo-2,5-diethoxy-4-morpholinobenzene, 1-diazo-2,5-diethoxy-4-p-tolylmercaptobenzene, 1-diazo-3-ethoxy-4-N-methyl-N-benzylaminobenzene, 1-diazo-3-chloro-4-N,N-diethyl-aminobenzene, 1-diazo-3-methyl-4-pyrrolidinobenzene, 1-diazo-2-chloro-4-N,N-dimethylamino-5-methoxybenzene, 1-diazo-3-methoxy-4-pyrrolidinobenzene, 3-methoxy-4-diazodiphenylamine, 3-ethoxy-4-diazodiphenylamine, 3-(n-propoxy)-4-diazodiphenylamine or 3-(isopropoxy)-4-diazodiphenylamine with a condensation agent, such as aliphatic aldehydes (preferred are formaldehyde, acetaldehyde, propionaldehyde and butyraldehyde), benzaldehyde or condensable aromatic compounds, in particular from phenol ethers, such as 4,4xe2x80x2-bis-methoxymethyl diphenyl ether. Especially advantageous examples of diazonium polycondensation products are reaction products of diphenylamine-4-diazonium salts, optionally having a methoxy group in the phenyl group bearing the diazo group. Aromatic sulfonates (such as 4-tolylsulfonate or mesitylene sulfonate) or dihydrogenphosphate are particularly suitable as anions of these diazo resins. Preferably the diazo resin is the condensation product of formaldehyde with diphenylamine-4-diazonium salt, more preferably said condensation product was isolated as organic sulfonate.
Instead of incorporating a diazo resin into the radiation-sensitive composition it is also possible to prepare a radiation-sensitive element by applying between substrate and radiation-sensitive layer a separate layer comprising a diazo resin. If the diazo resin is used as a subcoat it is preferred to use a condensation product of 3-methoxy-diphenylamine-4-diazonium sulfate and 4,4xe2x80x2-bis-methoxymethyldiphenyl ether preferably precipitated from an aqueous solution as dihydrogen phosphate.
Dependent on the substrate to be used for the preparation of the radiation-sensitive element the diazo resin is incorporated into the radiation-sensitive layer or applied as a separate subcoat. It is well known to a person skilled in the art that the adherence of a radiation-sensitive layer on a substrate depends on the kind of substrate especially the kind of pretreatment of the substrate; it is commonly known that the adherence to particular substrates can be improved by the presence of a diazo resin in the radiation-sensitive layer or as a subcoat. In some cases the use of a separate subcoat instead of incorporating the diazo resin into the radiation-sensitive layer improves the shelf life of the radiation-sensitive element.
Furthermore, the diazo resin can be present both in a radiation-sensitive element as a subcoat and as a component of the radiation-sensitive composition.
The radiation-sensitive compositions of the present invention are preferably usable for producing lithographic plates. In addition, however, they may be used in recording materials for creating images on suitable carriers and receiving sheets, for creating reliefs that may serve as printing molds, screens and the like, as light-hardening varnishes for surface protection and for the formulation of UV-hardening printing inks.
For the preparation of lithographic printing plates, aluminum as the carrier is first grained by brushing in a dry state, brushing with abrasive suspensions or electrochemically, e.g. in a hydrochloric acid electrolyte. The grained plates, which were optionally anodically oxidized in sulfuric or phosphoric acid, are then subjected to hydrophilizing after treatment, preferably in aqueous solutions of polyvinyl phosphonic acid, sodium silicate or phosphoric acid. The details of the above-mentioned substrate pretreatment are well-known to the person skilled in the art.
The subsequently dried plates are coated with the radiation-sensitive compositions out of organic solvents and/or solvent mixtures in such a way that dry layer weights of preferably from 0.5 to 4 g/m2, more preferably from 0.8 to 3 g/m2 are obtained.
In few cases, the additional application of an oxygen-impermeable top layer to the photosensitive layer may be advantageous. The polymers suitable for the top layer include polyvinyl alcohol, polyvinyl alcohol/polyvinyl acetate copolymers, polyvinyl pyrrolidone, polyvinyl pyrrolidone/ polyvinyl acetate copolymers and gelatin. The layer weight of the oxygen-impermeable top layer is preferably 0.1 to 4 g/m2, and more preferably 0.3 to 2 g/m2. However, the lithographic plates produced using the photosensitive compositions of the present invention do have excellent properties even with no top layer.
The thus obtained lithographic plates are exposed and developed as known to the person skilled in the art. The developed plates are usually treated with a preservative (xe2x80x9crubber coatingxe2x80x9d). The preservatives are aqueous solutions of hydrophilic polymers, wetting agents and other additives.
For certain uses it is furthermore advantageous to increase the mechanical strength of the printing layers by means of a heat treatment or a combined use of heat and UV radiation. For this purpose, the plate is first treated with a solution that protects the non-image areas such that the heat treatment will cause no ink receptivity in these areas. A suitable solution is described e.g. in U.S. Pat. No. 4,355,096. However, the lithographic plates produced using the photosensitive compositions of the present invention do have excellent properties even with no heat treatment.